The present invention relates generally to crystal growing apparatus used in growing monocrystalline silicon ingots, and more particularly to an electrode assembly for conducting electrical current to an electrical resistance heater used in such a crystal growing apparatus.
Single crystal silicon, which is the starting material for most semiconductor electronic component fabrication, is commonly prepared by the so-called Czochralski ("Cz") method. The growth of the crystal is most commonly carried out in a crystal puller. In this method, polycrystalline silicon ("polysilicon") is charged to a crucible and melted by a heater surrounding the outer surface of the crucible side wall. A seed crystal is brought into contact with the molten silicon and a single crystal ingot is grown by slow extraction via a crystal pulling mechanism. After formation of a neck is complete, the diameter of the crystal ingot is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal, which has an approximately constant diameter, is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the pull rate and heat supplied to the crucible. When the diameter becomes small enough, the ingot is then separated from the melt.
Commonly used heaters for melting silicon in the crucible are electrical resistance heaters in which an electrical current flows through a heating element constructed of a resistive heating material (e.g., graphite). The resistance to the flow of current generates heat that radiates from the heating element to the crucible and silicon contained therein. The heating element comprises vertically oriented heating segments of equal length and cross-section arranged in side-by-side relationship and connected to each other in a serpentine configuration. That is, adjacent segments are connected to each other at the tops or bottoms of the segments in an alternating manner to form a continuous electrical circuit throughout the heating element. The heating power generated by the heating element is generally a function of the cross-sectional area of the segments.
These heaters are mounted generally at the bottom of the crystal puller housing, which is constructed of steel. An electrode assembly includes a copper electrode that extends up through the bottom of the housing for electrical connection with the heating element of the heater. The other end of the electrode is electrically connected to power cables external of the housing so that electrical current drawn by the cables from a source of electrical current is conducted through the electrode to the heating element for operation of the electrical resistance heater. As the heating element radiates heat, the electrode is exposed to severe thermal loading. Since copper cannot withstand high thermal loads, the copper electrodes are constructed as tubes having a central cooling passage through which cooling water is directed along the length of the electrode to cool the electrode. As such, the diameter of the electrode must be relatively large to allow both sufficient mass to carry the necessary electrical load and passage for the cooling water.
It has recently been discovered that the quality of crystal ingots grown according to the Czochralski growth method can be improved by controlling the axial thermal gradient of the ingot as it cools during its ascent through the growth chamber and upper pull chamber. The thermal gradient is controlled using heat shielding and/or a second electrical resistance heater mounted in the upper pull chamber of the housing adjacent the dome-shaped transition portion. The heater includes a heating element that extends downward into the crystal growth chamber, terminating substantially above the crucible containing the molten source material. A central opening of the heating element allows the growing ingot to pass centrally through the heating element as it is pulled upward through the housing of the puller. Mounting brackets are electrically connected to the top of the heating element for mounting the heater on the wall of the upper pull chamber. Openings in the wall of the upper pull chamber allow the mounting brackets to be electrically connected to a source of electrical current external of the housing.
Because of radial space limitations present in conventional crystal pullers, installing a second heater in the upper pull chamber and connecting the heater to a source of electrical current presents a difficult challenge. For example, the diameter of the upper pull chamber in crystal pullers used for growing ingots having diameters up to 200 mm is typically about 350 mm. This leaves only a small amount of radial spacing for the heater and a suitable electrode assembly for establishing an electrical circuit between the heating element and the source of electrical power and for adequately cooling the electrode. Mounting the second heater in the upper pull chamber of existing crystal pullers requires an expensive adapter disposed between the upper pull chamber and the dome-shaped transition portion of the housing. The expense and complexity of the adapter increases as the height of the adapter increases. Thus, it is desirous to minimize the required height of the adapter.
Conventional water-cooled copper electrode assemblies, such as that described above for use in supplying the crucible heater with electrical current, are undesirable for use with a heater mounted in the upper pull chamber of the crystal puller. The need to provide cooling water internally throughout the electrode results in an electrode having a significant diameter to allow both sufficient mass to carry the necessary electrical load and passage for the cooling water. The overall cross-sectional dimension of the electrode is thus also significant, thereby increasing the height requirement of the adapter. In addition, the water-cooled copper electrode consumes considerable space in locating it within the upper pull chamber wall for connection with the heating element.